PROJECT SUMMARY/ABSTRACT Dyslipidemia and insulin resistance predispose individuals to development of diabetes, cancer, myocardial infarction, and stroke. A large body of evidence suggests that a class of toxic lipids, termed ceramides, contribute to these metabolic impairments and the ensuing development of these metabolic disorders. Understanding the role of these lipids in the events that drive metabolic diseases holds great promise for developing new therapies to treat these debilitating conditions. We conducted a series of studies ablating the enzymes required for their production in different body locales to discern which tissues were most sensitive to ceramides. These studies revealed that the lipid has strong and unanticipated effects in adipose tissue. In particular, whole-body, adipose and brown adipose tissue-specific inhibition/deletion of serine palmitoyltransferase (Sptlc), the first enzyme in the enzymatic cascade that drives sphingolipid biosynthesis, in mice markedly altered adipose morphology and metabolism, particularly in subcutaneous and brown adipose tissue. We subsequently excised another gene in the pathway (i.d. dihydroceramide desaturase-1 (Degs1)) from adipose tissue, determining that it elicited a similar spectrum of metabolic improvements. These data indicate that ceramides serve as signals of nutrient excess that alter the metabolic activity of mature adipocytes and subsequently the entire organism. Using microarray screens, we sought to identify ceramide-regulated genes in adipose tissue. The candidate obesity gene Fgf13 was one of the only two transcripts that met the following criteria: (a) increased in mouse subcutaneous white adipose (sWAT) and epididymal white adipose (eWAT) after high fat feeding (HFD); (b) decreased in these depots when the mice were treated with the SPT inhibitor myriocin; (c) decreased in these depots following WAT-specific Sptlc2 depletion; and, (d) decreased in primary adipocytes following myriocin treatment in vitro. We then investigated the function of this protein in vitro and in vivo. Preliminary data using knockdown or knockout approaches suggested that Fgf13 had cell-autonomous, adipocyte-specific, diet-regulated effects on mitochondrial function and thermogenesis. Moreover, mice lacking Fgf13 in adipocytes were resistant to obesity. These data support our hypothesis that FGF13 is a ceramide effector that controls the metabolic activity of mature adipocytes. We will test this idea with the following Specific Aims: 1] to determine the role of FGF13 as a modulator of adipose tissue metabolism and thermogenesis in vivo; 2] to determine the molecular mechanisms linking FGF13 to adipocyte metabolism; and 3] to determine the molecular mechanisms by which b-adrenergic agonists regulate ceramide production and FGF13 expression in primary adipocytes. The findings obtained from these studies could reveal a novel ceramide effector that influences metabolic rate.